The United States Navy maintains a superior global Anti-Submarine Warfare (ASW) capability with the ability to detect, localize, identify, and track potentially hostile submarines. On a typical ASW mission, a number of expendable sonobuoys are deployed from an aircraft. The Sonobuoys generally provide both an acoustical signal source and a reception capability for underwater acoustic signals of interest. Parameters affecting acoustic signals, for example, depth, water temperature and salinity, may also be detected. This data is transmitted on uplink channels to monitoring units that process the signals for target analysis, classification, and recording for replay and post-event analysis. Established sonobuoy tactics allow for short and long range detection of surface ships and submarines resulting in the prosecution of identified hostile targets.
Presently a large number of ASW aircraft use sonobuoy receivers that have been in service for a number of years, for example, the AN/ARR-78 and modifications, as well as the ARR-78, ARR-84, ARR-78 v3. ARR-89, and modifications of those units (collectively referred to herein as “legacy sonobuoy receivers.”) The legacy sonobuoy receivers contain twenty or four receiver modules, depending on the model, each capable of accepting operating channels 1-99 (those sonobuoy channels now in use and those being developed for future use) in the VHF band from 136 to 173.5 megahertz (MHz). The receiver modules may be tuned to any one of the sonobuoy operating channels. The output from the legacy sonobuoy receivers is fed to a data demultiplexer which is implemented on a personal computer referred to as a Low-Cost Advanced Processor (LCAP) and which transforms the receiver frequency shift keyed (FSK) output to a digital stream that can be processed with a conventional computer. The LCAP output is passed to an AN/UYS-1 or AN/UYS-1A single advanced signal processor (SASP) system. The SASP system includes a spectrum analyzer and programmable signal processor that extracts and conditions acoustic sonobuoy data to determine frequency, amplitude, bearing, Doppler, range, and other characteristics for detection and tracking of underwater targets.
While the legacy sonobuoy receivers are highly reliable, they have certain inherent limitations that restrict the insertion of new technology. In particular, reception of FSK modulated digital transmissions from recently developed sonobuoys, for example, the AN/SSQ-101 and AN/SSQ-110 (referred to herein as “digital sonobuoys”), have presented new challenges. Although the legacy sonobuoy receivers are able to demodulate signals from the digital sonobuoys and can do so at the highest data rate of 256 kbps, they cannot perform any operations to reduce the bit error rate (BER). Systems that use digital sonobuoys require a BER of 10−5 (1 error in 100 Kbits), or better.
In traditional deep water ASW operations, radio frequency interference (RFI) is not usually significant and the BER requirements of the new sonobuoy systems can generally be achieved with existing transmission schemes and legacy sonobuoy receivers. However, in recent years, naval operations have increased in littoral waters where RFI from both land-based and small boat sources is much more problematic. When RFI occurs in a digital sonobuoy uplink channel that is being monitored by an aircraft, the data becomes unusable because of the increased BER caused by RFI and signal propagation problems. At present, forty-seven wideband sonobuoy channels are available for digital sonobuoys operating at 256 kbps. Frequently, RFI in littoral waters is so severe that only a small number of channels are available for use. Thus, the aircraft cannot complete its mission.
Although a number of methods exist to mitigate RFI, including, for example, narrowband filtering, and spatial interference nulling, these techniques are not easily integrated with systems incorporating legacy sonobuoy receivers. Narrowband notch filtering could be provided by the addition of an external filter unit. However, a narrowband filter would be expensive, require installation space, add weight and power requirements to the aircraft and be difficult to interface properly with the legacy sonobuoy receivers. Likewise, spatial filtering would require one or more additional external antennas to be added to the aircraft as well as an external device and interface for the receiver which would also require installation space and add weight and power requirements to the aircraft. While RFI might also be reduced by transmitting more power, this option is impractical because it would add weight, size and power requirements to the sonobuoys. Error control coding (ECC) is also impractical because it can not be implemented using legacy sonobuoy receivers within the constraints of existing channel capacity.
What is needed is a practical, reliable, and inexpensive way to improve the BER of systems incorporating legacy sonobuoy receivers so that data fidelity requirements of newly developed digital sonobuoys can be met under a wide variety of operating conditions. Embodiments according to the present invention are directed to solving this need.